This invention relates generally to the field of variable geometry turbochargers and, more particularly, to shaft seal design for use with a unison ring actuator crank disposed within a center or bearing housing of a variable nozzle turbocharger to reduce leakage of hot exhaust gas from an adjacent turbine housing while not adding to the operating friction of the crank.
Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
In a turbocharger it is often desirable to control the flow of exhaust gas to the turbine to improve the efficiency or operational range of the turbocharger. Variable geometry turbochargers have been configured to address this need. A type of such variable geometry turbocharger is one having a variable exhaust nozzle, referred to as a variable nozzle turbocharger. Different configurations of variable nozzles have been employed in variable nozzle turbochargers to control the exhaust gas flow. One approach taken to achieve exhaust gas flow control in such variable nozzle turbochargers involves the use of multiple pivoting vanes that are positioned annularly around the turbine inlet. The pivoting vanes are commonly controlled by a unison ring and actuator crank to alter the throat area of the passages between the vanes, thereby functioning to control the exhaust gas flow into the turbine.
In order to ensure the proper and reliable operation of such variable nozzle turbochargers, it is important that any leakage of exhaust gas from the turbine housing be minimized and that the plurality of vanes be controlled in a manner that is without impairment, e.g., by friction or otherwise. Variable nozzle turbochargers, such as that disclosed in U.S. patent application Ser. No. 09/408,694 entitled VARIABLE GEOMETRY TURBOCHARGER having a common assignee with the present application, comprise an actuator crank having a shaft that is disposed through the center or bearing housing, and that includes one shaft end disposed within the turbine housing and attached to the unison ring, and an opposite shaft end that is disposed within the center housing and attached to a hydraulic actuating mechanism. The Shaft is rotatably supported within an opening through the center housing by one or more bushings.
The center housing actuator crank shaft opening is subjected to different turbocharger operating different temperatures depending on location within the center housing. For example, a portion of the opening disposed within the center housing away from the turbine housing is exposed to an operating temperature that is similar to that of the lubricating oil routed through the center housing for lubricating the turbine shaft. A portion of the opening positioned adjacent the turbine housing, e.g., forming a wall of the turbine housing in function, is exposed to the relatively higher temperature exhaust gas that is directed into and passed through the turbine housing. This temperature differential within the actuator crank shaft opening is know to produce a considerable thermal growth differential that could cause the shaft to rub or otherwise bind up in the center housing, thereby impairing efficient and dependable actuator crank actuation.
Conventional variable nozzle turbochargers have been designed to address this issue of differential thermal growth of the actuator crank shaft opening by designing the portion of the opening within the wall adjacent the turbine housing to have an oversized or enlarged diameter hole. The enlarged diameter hole is oversized to account for the thermal growth related radial displacement of the center housing wall section, thereby avoiding potential binding or otherwise impairing interference with the actuator crank shaft. While the use of an oversized opening hole through the center housing wall may act to address the issue potential actuator crank shaft impairment by thermal growth, it produces a large leak path for the potential passage of exhaust gas from the turbine housing.
In order to address this leak path, conventional variable nozzle turbochargers include a sealing system that has an effect of imposing unwanted friction on the actuator crank shaft. Typical oil actuation systems for actuating the actuator crank functions with a wide variation in pressure. Many engines have only five psi of oil pressure at ideal conditions. Therefore, it is important that the amount of friction imposed on the vane actuation system be kept at a minimum to permit movement with very low actuation forces. The frictional forces imposed by sealing systems of conventional variable nozzle turbochargers can impair vane actuation.
It is, therefore, desirable that a variable nozzle turbocharger be constructed having an actuator shaft seal that is configured in a manner that facilitates actuation of the actuator crank and related unison ring and multiple vanes via low actuation forces without unwanted friction, thereby providing improved vane actuation reliability when compared to conventional variable nozzle turbocharger designs. It is further desired that such actuator shaft seal be constructed in a manner that does not produce an undesired amount of gas leakage from the turbine housing.